1. Field of the Invention
The present invention relates generally to glass containers and, more particularly, to a system and method for tempering glass containers such as bottles, tumblers, and jars.
2. Description of the Related Art
Tempered glass is generally defined as glass (e.g., annealed or ordinary) that has been pre-stressed by heating it to a temperature at or above its softening point and forcing the glass to suddenly and rapidly quench under carefully controlled conditions. This tempering process produces tempered glass, which has highly desirable conditions of induced stress that result in additional strength, resistance to thermal stress, and impact-resistance, as compared to annealed or ordinary glass.
The basic principle employed in the tempering process is to create an initial condition of surface- and edge-compression. This condition is achieved by first heating the glass and then quenching the surfaces thereof rapidly. Such heating and quenching leaves the center of the glass relatively hot compared to the surfaces thereof. As the center then cools, the surfaces and edges of the glass are forced into compression. Wind pressure, missile impact, thermal stresses, or other applied loads must first overcome the compression before there is any possibility of fracture to the glass.
With respect to the heating step, it is known to use a hearth or lehr to heat glass that is to be tempered. Generally speaking, the lehr is a furnace and may be of a continuous-roller type, fixtured-roller type, or gas type. For example, a gas-type lehr has a plurality of blocks disposed beneath a plurality of radiant heaters. Typically, the glass is placed inside the lehr, where the glass is heated by conventional radiation and convection and conduction heat. The glass is moved along the blocks at a predetermined rate, which depends upon the thermal conductivity of the glass, to reach a temperature in the forming range of the glass. When the glass is at a temperature in such range (e.g., approximately 1200° F.), the glass is formed into a predetermined shape of the blocks.
Once so formed, the surfaces of the glass are rapidly air-quenched, typically by application of an air stream thereto, thus creating a desired temperature differential or gradient between the center of the glass and the surfaces thereof to create a desired internal stress. The air stream can consist of arrays of fixed, reciprocating, or rotating nozzles. It is important to extract heat uniformly from all surfaces of the glass (uneven heat extraction may produce bow or warp) and to sustain the quench long enough to prevent reheating of the surfaces from the still-hot center of the glass. A quenched condition becomes stable when the glass is reduced to a temperature of approximately 400-600° F.
In the case of tempering glass containers such as glass bottles, however, the conventional tempering process just described is not practical. More specifically, known mechanisms designed and employed to quench the surfaces of the glass container do not sufficiently quickly quench the surfaces such that a desired temperature differential or gradient between the center of the glass walls and the surfaces thereof to create the desired internal stress is not created.
Therefore, there is a need in the art for a system and method for tempering glass containers, in general, and a system and a method for heating glass containers during the heating portion of the tempering process while maintaining a desired temperature differential or gradient between the center of the glass walls of the containers and the surfaces thereof to create the required internal stress, in particular. There is also a need in the art for a system and method that tempers glass containers such as bottles rapidly, efficiently, and inexpensively.